Bending presses are machine tools of a type that is itself well known. The machine comprises, as shown in accompanying FIG. 1, a lower table 12 and an upper table 14 that is movable relative to the lower table 12. Usually, the lower table 12 is stationary and the upper table 14 is suitable for being moved towards the lower table 12 under drive from actuators V1 and V2 that act on the ends 14a and 14b of the upper table. Usually, the lower table 12 has its free edge 12a fitted with fastener means 16 for fastening bending matrices 18. In the same way, the edge 14c of the upper table 14 is fitted with fastener means 20 for fastening bending punches 22.
A metal sheet or lamination F is placed on the bending matrices 18 of the lower table 12. The sheet F may be of a length that varies widely depending on circumstances. Under drive from the pistons of the actuators V1 and V2, the punches 22 mounted on the upper table move towards the sheet placed on the matrices of the lower table 12. As soon as the punches 22 come into contact with the sheet F, force begins to increase within the sheet as the punches penetrate therein, initially in the elastic range and subsequently in the plastic range, thereby enabling the sheet to be bent permanently.
Because the force is applied to the upper table 14 by the actuators V1 and V2 acting on the ends 14a, 14b of the table 14, the linear load distributed between the two ends of the tables corresponds to the upper table being deformed along a line in the form of a concave arc with deformation maximas close to the midplane of the table. This means that for bending purposes, at the end of bending, the central portions of the punches 22 have penetrated into the sheet F less than have the end portions. If bending were to be performed on a matrix 18 that, itself, were to remain perfectly straight during bending, then the result would be that a workpiece would be obtained having a bend angle that was wider in its central portion than at its ends. Such a result is naturally unacceptable.
In order to remedy that drawback, various solutions have been proposed for the purpose of controlling these deformations at the edges of the tables in order to obtain a bend that is substantially identical over the entire length of the bent sheet.
Conventionally, those solutions involve providing slots, such as the slots 24 and 26 shown in FIG. 1, that are formed in the lower table 12 symmetrically about the midplane P′P of the press. Those slots 24, 26 then define a central zone 28 of the lower table 12 that does not have slots and that presents a length b, both of the slots 24 and 26 being of length a. With slots 24 and 26 of conventional type, i.e. that leave between them a slot-free portion 28 of length b, substantially parallel deformations are obtained for the edges of the upper and lower tables 14 and 12.
In order to direct the movement of the tables 12, 14 when forces are applied by the actuators V1, V2 for moving the movable table 14 vertically, the frame of the bending press conventionally includes two cheeks 70, 71 for guiding the movable table 14 laterally and for holding the table stationary 12 on the movement axis of the movable table 14, for the entire duration of application of the bending force of the metal sheet or lamination by the actuators V1, V2. The movable table 14 includes rollers so as to promote sliding against the opposite faces of the cheeks 70, 71 with which said table 14 interacts while it is moving. In addition, at the current time, the stationary table includes guide rails that act on the opposite faces of the cheeks 70, 71 when the edge of the table is deformed under the stress of the bending force while bending a metal sheet. These rails are made of a material that limits friction forces. They are generally made of machined steel, of bronze or of synthetic materials. In order to create accurate guidance, without any slack between the cheeks 70, 71 and the deformable portion of the table 12, and also to ensure the best possible rigidity, these guide rails are often subjected to prestress forces by means of spring washers and precise geometric adjustment of their positions.
However, the solution of using guide rails is not entirely satisfactory.
The large number of friction zones as well as the necessary prestressing of the guide rails means that relatively high forces are involved. As a result, the deformations at the edge of the table having slots are poorly controlled, and that may lead to inaccuracies in the positioning of the edge during bending of the metal sheet, especially when the slots are long. In addition, when the edge of the table having slots returns towards its non-deformed position, after bending, vibration may occur. In order to reduce this phenomenon, it is necessary to lubricate the friction faces of the guide rails and/or of the cheeks, and that presents in particular an additional maintenance cost.
In addition, the guide rails are specific parts of cost that is considerable and they may present wear, requiring them to be replaced.